This invention relates to a device for automatically controlling a diaphragm aperture, and more specifically to such a device for a single lens reflex camera, which is capable of automatically setting an aperture value so as to obtain a proper exposure based on exposure informations such as scene brightness, shutter speed, film sensitivity, etc.
It has previously been proposed to provide various devices for automatically controlling diaphragm a aperture of the type as above described, such as:
(1) a system for controlling the aperture by mounting a diaphragm blade on a moving coil for an exposure meter instead of the meter pointer,
(2) a system which relies on a servomotor and a servo-amplifier to balance a servo-system to obtain a proper aperture,
(3) a system for locating the meter pointer to limit a stop-down, and
(4) a system which is adapted to fix the diaphragm by using electromagnetic means when the output voltage of a potentio-meter in association with the diaphragm and the output voltage of a photometric circuit are equalized to each other upon comparison therebetween.
It has been customary to widely employ the system (3) or (4) due to the fact that still cameras are generally required to provide a short aperture control time. Disadvantages derived from the use of the system are, however, that a micrometer which is mostly actuated by a weak current lessens a mechanical reliability. Problems associated with the use of the system (4) are the need of a potentiometer with a high-tolerance, which is operated in cooperation with the aperture at a relatively high speed.
It is a primary object of the present invention to eliminate the aforementioned disadvantage of conventional device for automatically controlling diaphragm aperture and to improve such devices.
A further object of this invention is to provide a device for automatically controlling diaphragm aperture, simple in structure and inexpensive in manufacture, which is capable of ensuring its performance without necessity of parts with a high tolerance.
The present invention is characterized in that a pulse number generated by switch means opened and closed in association with a diaphragm drive, is applied to a first counter circuit, and that the output of a second counter circuit to which a pulse in number corresponding to the aperture value to be controlled has already been applied or the output of a latch circuit fixing such aperture number is compared by a digital comparator to the output of the first counter circuit, and that the diaphragm aperture is fixed by electromagnetic means or the like when the both informations coincide with each other.
More specifically, a pulse number is provided corresponding to a stop-down volume by an electromagnetic or mechanical switch or the like in cooperation with a stop-down to serve as an exposure information. This information is compared by a digital comparator with a pulse number corresponding to the aperture value to be set as fixed in a latch circuit or a counter circuit. The diaphragm aperture may be fixed by actuation of electromagnetic means with the output of a digital comparator when the both pulse numbers are coincident with each other upon comparison to obtain a proper aperture.
This arrangement has advantages in that the aperture information is provided without necessity of a potentiometer with a high tolerance. A control station is composed of only a digital circuit and is thus readily converted to an integrated circuit to render the structure simple.
The present invention will be now explained by reference to the following description of preferred embodiment of the invention taken in conjunction with the accompanying drawing.